fbdev: nvidiafb: Correct const string length in nvidiafb_setup()

The actual length of const string "noaccel" is 7, but the strncmp()
branch in nvidiafb_setup() wrongly hard codes it as 6.

Fix by using actual length 7 as argument of the strncmp().

Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Signed-off-by: Helge Deller <deller@gmx.de>